Scientists have discovered an ancient coastline 550 feet below the surface of the Black Sea, providing dramatic new evidence of a sudden, catastrophic flood around 7,500 years ago--the possible source of the Old Testament story of Noah.

A team of deep-sea explorers this summer captured the first sonar images of a gentle berm and a sandbar submerged undisturbed for thousands of years on the sea floor. Now, using radiocarbon dating techniques, analysts have shown that the remains of freshwater mollusks subsequently dredged from the ancient beach date back 7,500 years and saltwater species begin showing up 6,900 years ago.

Explorer Robert D. Ballard, who led the team that collected the shells, said the findings indicate a flood occurred sometime during the 600-year gap. "What we wanted to do is prove to ourselves that it was the biblical flood," Ballard said in an interview this week.

The findings offer independent verification of a theory advanced by Columbia University geologists William Ryan and Walter Pitman that the Black Sea was created when melting glaciers raised the sea level until the sea breached a natural dam at what is now the Bosporus, the strait that separates the Mediterranean Sea from the Black Sea.

An apocalyptic deluge followed, inundating the freshwater lake below the dam, submerging thousands of square miles of dry land, flipping the ecosystem from fresh water to salt practically overnight, and probably killing thousands of people and billions of land and sea creatures, according to Ryan and Pitman.

The two scientists described the catastrophe in their book "Noah's Flood," based on 30 years of research that began with coring samples showing the same abrupt transition from lake to sea that Ballard confirmed with his dredge. No one had ever actually seen the old shoreline, however, until Ballard's team captured sonar images of it in August.

Ryan and Pitman also suggested that the flood may have triggered massive migrations to destinations as diverse as Egypt, western Europe and central Asia, an idea that has provoked some academic controversy. Scholars also question whether any natural disaster could be conclusively identified as the inspiration for the story of Noah's flood.

"All modern critical Bible scholars regard the tale of Noah as legendary," said Hershel Shanks, editor of the Biblical Archaeology Review. "There are other flood stories, but if you want to say the Black Sea flood is Noah's flood, who's to say no?"

Shanks pointed out that biblical scholars date the writing of the Book of Genesis, from which the story of Noah is taken, at sometime between 2,900 and 2,400 years ago, and a similar event is described in the Mesopotamian Gilgamesh legend, written about 3,600 years ago.

But while Ryan and Pitman do not prove that the Black Sea flood directly inspired Gilgamesh or Noah, their theory argues persuasively that the event was probably horrific enough for scribes and minstrels to remember it for thousands of years.

And regardless of the historical context, the science of the Black Sea flood stands undisputed. Ryan and Pitman dated the event at 7,600 years ago, and they fixed the likely depth of the ancient coastline almost exactly where Ballard found it.

"It feels good," Pitman said of Ballard's findings, analyzed by the Woods Hole Oceanographic Institution in Massachusetts. Pitman noted that the new research took place on the Black Sea's southern shore near the Turkish port of Synope--far from the northern waters where he and Ryan had worked.

The flood, the underwater coastline and the likelihood that ancient settlements lie on the submerged plain have added a new dimension to an already ambitious project.

The region's main archaeological attraction has always been the Black Sea itself, composed mostly of dense Mediterranean salt water that immediately plunged to the bottom of the freshwater lake when the Bosporus gave way 7,500 years ago.

Ever since, the less dense water on top has acted as a 500-foot-deep lid on a 7,000-foot-deep oxygen-free abyss--a watery wilderness where scientists suspect there may be 7,500 years of shipwrecks preserved in almost pristine condition.

The tantalizing prospect of exploring this environment piqued Ballard's interest several years ago. Beginning with the Titanic in 1985, Ballard has found several historic wrecks in deep water using manned submersibles and robotic vehicles.

The Black Sea project, funded by the National Geographic Society and the University of Pennsylvania, began in 1995, when teams of archaeologists on land and in shallow water began mapping Synope and its environs.

Synope is about 200 miles directly south across the Black Sea's abyssal waters from the Crimea--a natural terminus for an ancient trade route. Ballard said he intends to use a deep-sea robot next summer to look for a sea lane.

"The first thing you find is trash; you didn't have Adopt-a-Highway then," he said. And where there is trash, there are sure to be wrecks. "My biggest problem is going to be trees," he added. If wooden ships can survive in the Black Sea's depths, then so can trees. The bottom could look like a forest.

These difficulties, Ballard said, are different from those inherent in the search for flood-plain settlements. Many of these were probably buried--and lost forever--when a thick layer of sediment swept into the old lake with the flood waters. And Ballard suspects many others have been destroyed by the trawlers that have been scouring the sea bottom for thousands of years.

Still, he said, there are plenty of "relic surfaces" near Synope, where the water simply rose quickly to submerge intact whatever lay below. Ballard's sonar sweeps this summer found a gentle coastline "frozen in time," he said.

"In a perfect world you'll see a fence," Ballard said, or maybe a stockade or even a house. And there will likely be plenty of artifacts, because "when the flood came, people just had to run."